Method of making a precision electric fuse



Sept. 17, 1968 R. c. RAGAN 3,401,452

METHOD OF MAKING A PRECISION ELECTRIC FUSE Original Filed April 14, 1964 2 Sheets-Sheet 1 FIG. 1

INVENTOR.

Randal/C Raga 7 Sept. 17, 1968 R. c. RAGAN 3,401,452

METHOD OF MAKING A PRECISION ELECTRIC FUSE Original Filed April 14, 1964 2 Sheets-Sheet 2 INVENTOR.

Randal/ C Raga/7 United States Patent 0 5 Claims. c1. 29-623) The present invention relates generally to electrical fuses and, more particularly, to an improved precision electrical fuse.

This application is a divisional of my copending application Ser. No. 359,569, filed Apr. 14, 1964, now Patent No. 3,271,544 which application is a continuation-impart of my copending application Ser. No. 779,605, Precision Electrical Circuit Elements, filed Dec. 11, 1958, which in turn is a continuation-in-part of my application Ser. No. 618,728, Precision Electrical Circuit Elements, filed Oct. 29, 1956, now abandoned.

It is a primary object of this invention to provide an improved miniature electrical fuse which produces a fast and decisive break at the rated breaking current. A related object is to provide such a fuse which completely eliminates any arcing across the blown fuse element. Thus, it is another object to provide such a fuse which may be used in high voltage circuits with assured reliability of current cut off. Still another object is to provide such a fuse which fires fast enough to protect rapidly responding semiconductor devices.

It is another object of the invention to provide an improved miniature electrical fuse which is precise and acu curately reproducible for the most exacting applications, and yet is inherently simple and economical to manufacture. A connected object is to provide such a fuse which can be produced in a wide range of current ratings.

It is a further object of the invention to provide a precision electrical fuse which is extremely stable, both electrically and physically, over extended periods of operation. Thus, it is an object to provide such a fuse which maintains its rated breaking current even when operated for extended periods at relatively high temperatures. Another object is to provide such a fuse which is highly resistant to corrosion and abrasion. Yet another object is to provide such a fuse which is capable of withstanding current spikes of short duration without changing its rated breaking current.

A still further object of the invention is to provide an improved miniature electrical fuse which can be used in multiple arrangements in either series or parallel. In this connection, it is an object to provide such fuses which, when connected in series in high voltage circuits, distribute the voltage evenly among the series connected units. A related object is to provide such fuses which can be conveniently assembled in multiple arrangements on a single compact substrate.

Other objects and advantages of the invention will become apparent upon reading the following description and appended claims and upon reference to the drawings, in which:

FIGURE 1 is a plan view of a miniature electrical fuse embodying the present invention;

FIG. 2 is a sectional elevation view showing the fuse of FIG. 1 housed in a suitable fuse capsule;

FIG. 3 is a perspective view of a series arrangement of fuses similar to the fuse of FIG. 1 formed on a single substrate;

FIG. 4 is an exploded view of the series arrangement of fuses shown in FIG. 3;

3,401,452 Patented Sept. 17, 1968 FIG. 5 is a sequence of enlarged fragmentary plan views illustrating the various steps of a preferred method of forming the series arrangement of fuses shown in FIGS. 3 and 4;

FIG. 6 is a sequence of perspective views illustrating the various steps involved in forming a modified series arrangement of fuses embodying the invention; and

FIG. 7 is a fragmentary perspective view of the final product made according to FIG. 6 and mounted on an insulating substrate.

While the invention will be described in connection with a preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment but, on the contrary, it is intended to cover the various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Turning now to the drawings, in FIGS. 1 and 2 there is shown a miniature electrical fuse including an elongated substrate 10 formed of an electrically insulating material, a pair of spaced apart electrically conductive fuse electrodes 11 and 12 supported on the substrate 10 in intimate contact therewith, and a fuse element 13 extending between and electrically connecting the leads 11 and 12. In order to insure a rapid and decisive electrical break in the fuse element 13 when the current exceeds the rated maximum load, the substrate 10 should have a low thermal conductivity and high surface and volume electrical resistivity, both before and after the blowing of the fuse element. The low thermal conductivity minimizes heat losses from the fuse element to the substrate. Thus, when an overload in the electric circuit increases the temperature of the fuse element, practically all the heat is retained in the fuse element, and the fuse is heated to its blowing point instantaneously. The high surface and volume electrical resistivity of the substrate concentrates all the effective resistance of the fuse in the fuse element 13, and insures a complete and decisive electrical break upon blowing of the fuse element, with no residual conductivity in the substrate. In adidtion to the low thermal conductivity and high electrical resistivity, the substrate should have a smooth surface so as to permit the deposition of substantially uniform films thereon.

The preferred substrate material is glass, such as sodalime glass or lead-boro-silicate glass. It has been found that these glasses have high surface and volume resistivity and provide excellent thermal insulation for the fuse element, even when used as extremely thin wafers having a thickness of approximately 0.030 inch for example. Moreover, these glasses provide a smooth surface for film deposition and are low in cost and easy to fabricate. A typical glass which has been found to be especially useful as a substrate material in this invention is sodalime glass having a composition as follows:

Percent Silicon dioxide 71.96 Iron oxide 0.037 Aluminum oxide 1.42 Calcium oxide 8.11 Magnesium oxide 4.23

Sodium oxide 13.62 Potassium oxide 0.29 Sulfur trioxide 0.29

Alternatively, the substrate 10 may be made of any other suitable thermally stable insulating material such as, for example, electrical porcelain, seatite, fosterite, mica and other ceramic materials having the necessary chemical, electrical, and physical properties for the particular use intended. The substrate must, of course, be capable of withstanding the action of any chemicals used in the deposition of the electrodes 11 and 12 and the fuse element 13, as well as any changes in temperature encountered during manufacture and use of the fuse. Most organic plastics and certain ceramics such as alumina and beryllia are not satisfactory substrate materials because of their comparatively high thermal conductivity.

The fuse gap which is occupied by the fuse element 13 is formed by the spaced apart fuse electrodes 11 and 12 supported on the surface of the substrate 10. In order to concentrate substantially all the effective resistance of the fuse in the fuse element 13, the electrodes 11 and 12 should be made of a highly conductive metal so that their electrical resistance is as low as possible. If the leads 11 and 12 have any substantial electrical resistance, they represent another variable to be considered in manufacturing and testing the fuse, and also alter the time-temperature curve of the finished fuse,

The preferred electrode material is silver, because it requires the minimum amount of material for any given conductivity, but any other suitable highly conductive metal may be employed. When silver is used for the entire electrode, however, it has been found to have a deleterious effect upon the blowing of the fuse, as described in more detail hereinafter. Thus, in the preferred embodiment of FIG. 1, each electrode 11 and 12 is composed of a base layer 14 of silver and an overlay 15 of gold. The fuse gap between the two electrodes is defined by a pair of curved edges 15a which facilitate adjustment of the width of the fuse gap and also provide a convergent path of high conductivity. It should be noticed that the curved portions of the gold overlays 15 protrude beyond the silver layers 14, thereby providing a pair of gold electrode terminals which are compatible with proper operation of the fuse. In general, the silver should be kept at least about 0.025 inch away from the fuse element.

In the practice of the present invention, there is provided a composite metal-glass fuse element comprising an electrically conductive fuse metal extending between and electrically connecting the spaced apart fuse electrodes, and a layer of electrically insulating glass covering the metal layer in intimate contact therewith for quenching the fuse metal after it has been blown by a current overload, whereby any arcs which tend to develop across the blown metal are effectively suppressed. Thus, referring to FIGS. 1 and 2, the fuse element 13 includes a metal layer 16 extending between and electrically connecting the electrodes 11 and 12, and a glass coating 17 which completely covers the metal 16 in intimate contact therewith. In the particular embodiment illustrated, the glass coating also covers the adjacent end portions of the electrodes 11 and 12, and is bonded to the substrate 10 on both sides of the metal 16.

This invention stems in part from the unexpected discovery that if a composite metal-glass fuse element is made from certain compatible metal and glass components, the glass component completely suppresses any arcs which normally tend to develop after the blowing of a fuse in a high voltage circuit, without any inhibiting effect whatever on the rapid blowing of the fuse metal. Indeed, the glass coating has actually been found to increase the firing speed of the fuse. This effect is especially surprising when one considers that the fuse metal is completely encased at the time of firing, i.e., there are no voids into which the fuse metal can be blown. Although the explanation for this phenomenon is not entirely clear, it is believed that the blowing of the fuse softens the glass coating and permits the blown metal to migrate or be absorbed within the glass coating. Consequently, little or no metal vapor remains within the fuse junction, and there is no opportunity for a conductive plasma or highly ionized gas path to develop between the fuse electrodes. Furthermore, the softening of the glass permits migration of the metal into the glass thereby slightly derating the unit so that it breaks decisively. This effect, combined with the added thermal insulation provided by the glass coating, leads to a rapid firing rate so that the fuse does not hang on at a current near the breaking point.

In order to insure that arcing is completely suppressed upon blowing of the fuse metal, it is important that the metal and glass components of the composite fuse element be such that the blown metal readily migrates into the glass coating, thereby effectively preventing subsequent vaporization and ionization of the blown metal. The preferred metal component is gold. Gold not only migrates readily into most low melting glass coatings, but also has a high positive temperature coefficient of electrical resistivity and, therefore, blows rapidly under overload conditions. In other words, the gold increases in resistivity as the temperature is increased, thereby providing an avalanche or snowballing effect which produces substantially instantaneous blowing. In addition, gold has many properties which are desirable in the manufacture and normal operation of the fuse. Thus, for example, gold has good electrical conductivity, does not oxidize, is chemically and electrically stable, and is compatible with the ceramic processes which are preferably used in the manufacture of the fuse.

In contrast with gold, silver has been found to be completely useless as a metal component in the compo-site fuse element of this invention, in spite of the fact that silver is generally considered a close relative of gold. Indeed, when silver is used as the fuse metal in this invention and is blown by a current overload, it causes the surrounding glass to become highly conductive and to support higher and higher current until sufficient heat is generated to melt all parts of the circuit in the immediate vicinity of the fuse. In some cases, the silver actually causes an explosion. It is for this reason that the silver electrodes must be provided with terminals made of gold or other nondeleterious metal which keep the silver at least about 0.025 inch away from the fuse element, as described above.

Another metal which may be used as the metal component of the composite metal-glass fuse element is platinum. Although platinum has the disadvantages of a higher vaporization temperature and higher electrical resistivity than gold, it has been found that platinum will blow and migrate into a glass coating the same as gold when subjected to a current overload.

It will be recognized that the size and shape of the metal component of the composite metal-glass fuse may be adjusted to tailor the fuse to particular operating conditions and to provide different time-current curves. For example, a relatively thin and wide metal film covering a large substrate area will have greater immunity to current spikes of short duration than a relatively thick and narrow film covering a smaller substrate area, because of the difference in the heat dissipation capacities of the two types of film. Of course, the exact size and shape of the metal film required in any given fuse depend not only on the particular characteristics desired in the final fuse, but also on the particular materials employed. In the case of pure gold used as the fused metal in a fuse suitable for most conventional applications, a metal layer 0.005-inch square may be .0006 inch thick in a S-am'pere fuse and 0.002 inch thick in a -am-pere fuse.

In accordance with one aspect of this invention, it has been found that the response time of the fuse, i.e., the time required for the fuse to blow when subjected to a current overload, generally decreases as the length and/ or the width of the fuse metal layer is decreased. Also, the minimum response time for any given fuse is achieved when the length and width of the exposed portion of the fuse metal layer are substantially equal, as illustrated in FIG. 1. For example, in a typical fuse rated at 3 to 5 amperes, the metal layer 16 is suitably five square.

In order to provide a precise and accurately roproducible fuse, the two electrodes 11, 12 and the fuse film 16 should be deposited with consistently uniform thicknesses and compositions. This may be accomplished by a number of suitable film deposition methods such as, for example, thermal decomposition of metal-containing compounds, electrodeposition, vacuum exaporation, cathode sputtering and the like. One particularly preferred method of depositing the metal films is by thermal decomposition of metal resinates. In this method, the resinate is initially deposited in liquid form, being applied to the desired areas of the substrate by a suitable stencilling technique, such as silk screening for example, or by the application of ordinary printing, engraving, and lithographing techniques and the like. After the liquid resinate has been deposited, it is heated to its decomposition temperature in an oxidizing atmosphere to drive off the volatile reaction products and deposit a solid metal film bonded firmly to the substrate. This method permits accurate control of the thickness and other properties of the metal film and is capable of depositing extremely thin films which are uniform and continuous. The exact thickness of the various films will, of course, vary with different fuses, but the thickness of the fuse film 16 is generally in the range of about 0.0005 to 0.005 inch. In certain cases where it is desired to increase the conductivity of the film deposited from the resinate, such as in forming gold overlays or terminals on silver-electrodes for example, fine metal powder, such as gold powder for example, may be mixed with the liquid resinate. Also, it is often desirable to metallize the substrate surface, as with a thin silver coating, prior to the application of the resinate for the purpose of providing an electrically conductive and mechanically stable base for the subsequent welding of wire leads to the fuse.

In order to insure that the blown fuse metal migrates quickly into the arc-suppressing glass coating 17, the coating is made of a low melting glass, i.e., an inorganic product of fusion cooled to rigidity without crystallization. More particularly, the coating 17 should be made of a glass having a melting point below that of the substrate 10. As in the case of the substrate 10, the glass coating 17 should also have a low thermal conductivity and high a surface and volume electrical resistivity, both before and after the blowing of the fuse metal 16. The low thermal conductivity minimizes heat losses from the fuse metal, while the high resistivity concentrates the effective resistance of the fuse in the metal layer 16.

In order for the composite fuse element 13 to remain useful over extended periods of operation, it is important that the particular material employed for the insulating coating 17 remains electrically nonconductive upon extended operation at relatively high temperatures. In this connection, it has been found that certain insulating materials are satisfactory when first applied, but tend to decompose or char and become conductive during use. This, of course renders the fuse useless even though the metal layer has not blown. Examples of such unsatisfactory insulating materials are Teflon, ethyl cellulose, and epoxy resin. Various powdered materials have also been tried as are suppressors, but the air spaces throughout the powdered material have been found to permit the formation of the conductive plasma which leads to destructive arcing in high voltage circuits.

The preferred material for the fuse coating 17 is a low melting point glass such as lead-boro-silicate glass (e.g., 128:1 ratio of B 0 PhD, and SiO respectively). This glass provides effective thermal and electrical insulation, remains stable over extended periods of operation at elevated temperatures, and readily absorbs the blown fuse metal. Other suitable glasses are the low melting sodalime glasses, phosphate glasses, and various modified leadboro-silicate glasses. Certain water soluble alkali silicate glasses, such as sodium silicate and potassium silicate glass, may also be used, provided they are thoroughly dried in order to reduce the moisture content to a satisfactorily low level to provide high electrical resistivity.

In order to prevent cracking and crazing of the glass coating 17 during alternate heating and cooling of the fuse either in the process of manufacture or in use, the coefficient of thermal expansion of the coating should be adjusted to correspond with the coetficient of thermal expansion of the substrate to which the coating is bonded. For example, in the cause of the preferred leadboro-silicate glass, the coefficient of thermal expansion may be adjusted by the addition of zirconium oxide.

The arc-suppressing glass coating 17 may be formed by printing, spraying, or otherwise depositing a finely ground glass flux over the previously deposited fuse metal 16, and then heating the glass flux to its fusing temperature. The fused flux forms a continuous impervious glassy coating which is bonded firmly to the substrate 10 on both sides of the metal 16. In order to achieve the desired glassy coating, the flux must be fired to its fusing point, but overfiring has been found to produce an uncontrollable increase in the resistance of the underlying metal. Also,, overfiring causes bubbling or blistering of the glass which may destroy the continuity of the arc-suppressing coating. The firing temperature and time employed in any given case depends on the particular coating composition used, and it will be understood that a number of different firing procedures may be devised for any given coating. For example, in the case of the preferred leadboro-silicate (1:8:1) glass coating, the flux may be fired at about 830 F. for a period of about 5 to 10 minutes. The thickness of the coating will vary for different applications, but in general the glass coating should have a thickness of at least about one mil, preferably at least 2 to 3 mils.

In one aspect of the invention, the voltage rating of the fuse is increased by providing a plurality of the composite fuse elements in series with each other, preferably on a single substrate, as illustrated in FIGS. 3-5. In a preferred method of forming this construction, an. elongated glass substrate 20 is initially provided with a plurality of small spaced apart films 21 of silver, as shown in FIG. 5a, by the thermal decomposition of silver resinate. After the silver films 21 have been deposited and solidified, a plurality of corresponding gold overlay films 22 are deposited on top of the silver films. As shown in FIG. 5b, alternate pairs of the gold films 22 have opposed curved edges 220 which define a plurality of fuse gaps 23.

It will be appreciated that the silver films 21 and the gold films 22 form a plurality of highly conductive composite silver-gold electrodes having terminals 22a of pure gold. In order to achieve high conductivity in the terminals 22a of pure gold. In order to achieve high conductivity in the terminals 22a, the gold films 22 are preferably deposited from a mixture of gold resinate and fine gold powder. In the case of relatively high amperage devices, it may be necessary to deposit the gold in multiple coatings to achieve the desired conductivity. Alternatively highly conductive gold films may be deposited by electrodeposition.

After the deposition of the gold films 22 has been completed, each fuse gap 23 is inspected to insure that all the gaps are of the desired uniform width, typically about five mils. In actual production, the gaps are originally made somewhat less than the width actually required in the final product, and then adjusted to the exact required width by a scribing technique under a magnifying device. Next, the metal component of the composite metal-glass fuse element is deposited in the form of a narrow strip 24 of gold film which bridges the various fuse gaps 23, as shown in FIG. 50, so as to electrically connect the curved gold terminals 22a. The gold strip 24 is preferably deposited by thermal decomposition of gold resinate.

To complete the composite fuse element, a small strip 25 of low melting glass is deposited over each fuse gap and that portion of the gold strip 24 bridging the gap, as in FIG. 5d. The glass strips 25 form the arc-suppressing coatings of the respective fuses, and are firmly bonded to the substrate 20 on opposite sides of each fuse gap between the gold films 22. The glass strips 25 may be formed by stencilling a suitable glass flux onto the desired areas and then fusing the deposited flux to form smooth glassy coatings. This completes a multi-element series fuse ready for encapsulation in the manner described hereinafter.

A modified series arrangement of fuses which is especially adapted to carry relatively high currents and sunplify production procedures is shown in FIGS. 6 and 7. In this construction, a small gold wire 30 is initially provided with a plurality of annular plastic spacers 31 equally spaced along the axis of the wire. To provide the desired fuse electrodes, the wire 30 is electroplated or otherwise coated with a gold coating 32 which is broken only by the spacers 31. The plastic spacers are then dissolved by means of a suitable solvent, thereby providing a plurality of spaced fuse gaps 33 occupied only by the exposed portions of the gold wire. These exposed portions of the gold wire form the metal components of the series arrangement of composite fuse elements, while the highly conductive coated sections of the rod form the fuse electrodes. This entire assembly is then seated in a groove 34a of an insulating glass substrate 34, and each fuse gap 33 is coated with an arc-suppressing glass coating 35. It will be appreciated that the relatively large metallic cross section achieved by this construction provides a large current-carrying capacity.

In accordance with one aspect of this invention, a pair of leads is connected to the fuse electrodes, and the entire assembly is placed in a standard form package that will fit conventional circuit hardware. Thus, referring to FIG. 2, a pair of wire leads 40 and 41 is soldered to the fuse electrodes on the surface of a glass substrate 10, and then both the fuse and the leads are embedded in a solid insulating cylinder 42 with ends of the leads 40, 41 protruding from. the ends of the cylinder. This construction not only provides additional mechanical support for the leads 40, 41, but also insures that any arcs or other effects from an accidental failure of the arc suppressor are contained within the encapsulating package. The package is finished by fitting a pair of end caps 43 and 44 over the ends of the cylinder 42 and soldering them to the protruding ends of the wire leads.

In order for the insulating cylinder 42 to protect the fuse without affecting its operation, the cylinder 42 must have high dielectric-strength and high electrical resistance. In addition, the cylinder should be made of a material which is resistant to moisture, thermal shock, vibration, or any other environmental hazards which may be encountered in use. A preferred material for the encapsulating cylinder is epoxy casting resin, or an epoxy compression or transfer molding material, which may be formed in a molding shell fitted around the fuse assembly. Of course, it will be appreciated that the encapsulating package is not limited to the cylindrical form shown in the drawing, but may be formed in any suitable shape.

While various specific forms of the present invention have been illustrated and described herein in some detail, it will be understood that the same are susceptible of numerous modifications with in the spirit and scope of the invention. Thus, although the multiple arrangements of the subject fuse have been described with particular ref erence to series arrangements to increase the voltage ratings of the fuse, the invention is equally applicable to parallel arrangements of the composite fuse elements to increase the current-carrying capacity of the fuse.

It can be seen that this invention provides an improved miniature electrical fuse which produces a fast and decisive break at the rated breaking current. The effective thermal insulation provided on both sides of the fuse metal by the nonconductive substrate and the arc-suppressing coating, combined with high positive temperature coefficient of electrical resistivity of gold, produce an avalanche effect which causes practically instantaneous blowing at the breaking current. Moreover, the arc-suppressing coating completely eliminates any arcing across the blown fuse, thereby providing a fuse which may be used in high voltage circuits with assured reliability of current cut off.

What is claimed is:

1. A method of producing a precision electrical fuse for protecting an electric circuit, said method comprising the steps of providing an electrically insulating substrate which has a low thermal conductivity and is resistant to high temperatures, depositing a pair of spaced apart electrically conductive fuse electrodes on the surface of said substrate in intimate contact therewith, depositing a thin layer of electrically conductive fuse metal selected from the group consisting of gold and platinum extending between and electrically connecting said electrodes, said fuse metal being adapted to blow when the electrical current therethrough exceeds a predetermined level and depositing a coating of low melting point electrically insulating glass covering said fuse metal in intimate contact therewith for quenching and absorbing the fuse metal as it is blown, thereby effectively suppressing any arcs which tend to develop between the fuse electrodes upon the blowing of said fuse metal.

2. A method of producing a precision electrical fuse for protecting an electric circuit, said method comprising the steps of providing a glass substrate having low thermal conductivity and high surface and volume electrical resistivity, depositing a pair of spaced apart silver electrode films on the surface of said substrate in intimate contact therewith, depositing a pair of spaced apart gold terminal films on the opposed ends of said silver films, depositing a thin film of electrically conductive fuse metal selected from the group consisting of gold and platinum extending between and electrically connecting said gold terminal films on the surface of said substrate, said fuse metal being adapted to blow when the electrical current therethrough exceeds a predetermined level, covering said fuse metal with a coating of lead-boro-silicate glass for quenching and absorbing the fuse metal as it is blown whereby any arcs which tend to develop across the blown fuse metal are effectively suppressed, said lead-boro-silicate glass coating having a coefficient of thermal expansion corresponding with the coefficient of thermal expansion of said glass substrate, and providing a pair of electrical leads connected to said silver films for connecting the fuse into the electric circuit to be protected.

3. A method of producing a precision electrical fuse for protecting an electric circuit, said method comprising the steps of providing a glass substrate having low thermal conductivity and high surface and volume electrical resistivity, depositing a pair of spaced apart electrically conductive fuse electrodes on the surface of said substrate in intimate contact therewith, depositing a thin layer of electrically conductive fuse metal selected from the group consisting of gold and platinum extending between and electrically connecting said electrodes, said fuse metal being adapted to blow when the electrical current therethrough exceeds a predetermined level, the length and width of said fuse metal being substantially equal, depositing a coating of low melting electrically insulating glass covering said fuse metal in intimate contact therewith for quenching the fuse metal as it is blown and preventing subsequent vaporization of said metal whereby any arcs, which tend to develop between the fuse electrodes are effectively suppressed, said coating being bonded to said substrate on opposite sides of said fuse metal, at least the end portions of said fuse electrodes being made of a metal which does not substantially increase the electrical conductivity of said coating upon the blowing of said fuse metal, and providing a pair of electrical leads connected to said fuse electrodes for connecting the fuse into the electric circuit to be protected.

4. A method of producing a precision electrical fuse for protecting an electric circuit, said method comprising the steps of providing an elongated glass substrate having low thermal conductivity and high surface and volume electrical resistivity, depositing a plurality of pairs of spaced apart electrically conductive fuse electrodes deposited on the surface of said substrate in intimate contact therewith, the opposed edges of alternate pairs of said electrodes having gold terminals thereon to define a plurality of predetermined fuse gaps in series along the surface of said substrate, depositing a thin layer of electrically conductive fuse metal selected from the group consisting of gold and platinum within each of said fuse gaps so as to extend between and electrically connect said gold terminals, said fuse metal being adapted to blow when the electrical current therethrough exceeds a predetermined level, and covering said fuse metal with a layer of low melting point electrically insulating glass at each fuse gap for quenching and absorbing the fuse metal as it is blown, whereby any arcs which tend to develop between the fuse electrodes upon the blowing of said fuse metal are effectively suppressed.

5. A method of producing a precision electrical fuse for protecting an electric circuit, said method comprising the steps of providing an elongated gold wire, forming a plurality of annular spacers titted over said wire at equally spaced positions along the axis thereof, forming an elec- 20 trically conductive metal coating around the exposed portions of said wire between said spacers, removing said spacers so as to expose small portions of said wire between the coated portions thereof, thereby providing a series of metal fuse elements adapted to blow when the electrical current therethrough exceeds a predetermined level, mounting said coated wire on an elongated glass substrate having low thermal conductivity and high surface and volume electrical resistivity, and depositing a coating of low melting point electrically insulating glass over said metal fuse elements for quenching and absorbing said elements as they are blown, whereby any arcs which tend to develop upon the blowing of the fuse are effectively suppressed.

References Cited UNITED STATES PATENTS 2,576,405 11/1951 -McAlister 200l3l 3,023,289 2/1962 McAlister 200--131 FOREIGN PATENTS 499,816 1/ 1939 Great Britain.

JOHN F. CAMPBELL, Primary Examiner.

I. L. CLINE, Assistant Examiner. 

2. A METHOD OF PRODUCING A PRECISION ELECTRICAL FUSE FOR PROTECTING AN ELECTRIC CIRCUIT, SAID LMETHOD COMPRISING THE STEPS OF PROVIDING A GLASS SUBSTRATE HAVING LOW THERMAL CONDUCTIVITY AND HIGH SURFACE AND VOLUME ELECTRICAL RESISTIVITY, DEPOSITING A PAIR OF SPACED APART SILVER ELECTRODE FILMS ON THE SURFACE OF SAID SUBSTRATE IN INTIMATE CONTACT THEREWITH, DEPOSITING A PAIR OF SPACED APART GOLD TERMINAL FILMS ON THE OPPOSED ENDS OF SAID SILVER FILMS, DEPOSITING A THIN FILM OF ELECTRICALLY CONDUCTIVE FUSE METAL SELECTED FROM THE GROUP CONSISTING OF GOLD AND PLATINUM EXTENDING BETWEEN AND ELECTRICALLY CONNECTING SAID GOLD TERMINAL FILMS ON THE SURFACE OF SAID SUBSTRATE, SAID FUSE METAL BEING ADAPTED TO BLOW WHEN THE ELECTRICAL CURRENT THERETHROUGH EXCEEDS A PREDETERMINED LEVEL, COVERING SAID FUSE METAL WITH A COATING OF LEAD-BORO-SILICATE GLASS FOR QUENCHING AND ABSORBING THE FUSE METAL AS IT IS BLOWN WHEREBY ANY ARCS WHICH TEND TO DEVELOP ACROSS THE BLOWN FUSE METAL 